Stratospheric balloons are used for a wide range of science, military and commercial applications. In most of these applications longer duration flights are needed. One example is the Google Balloon Constellation program called “LOON.” The LOON program is a platform that can provide internet service to large areas of the world that presently do not have the needed telecommunications landline infrastructure. Hence, it requires balloons that can maintain flight for extended periods of time.
Presently there are two common types of stratospheric balloons, the zero-pressure balloon and the super-pressure balloon. Zero-pressure balloons are partially inflated with a lifting gas, with the gas pressure the same both inside and outside the balloon. As the zero-pressure balloon rises, the gas expands to maintain the zero pressure difference, and the balloon's envelope will inflate. More specifically, the balloon keeps ascending until the envelope reaches maximum volume and then, any excess lifting gas spills out a vent at the bottom of the balloon, resulting in the pressure of the lifting gas being equal to the atmospheric pressure at the target altitude. Since the maximum pressure in the balloon hull is only slightly higher than the atmospheric pressure, the balloon can be fabricated from relatively low strength ductile film like polyethylene. These balloons have proven to be relatively inexpensive and reliable. The problem with using zero-pressure balloons for extended flight periods is when the balloon is at float, the sun heats the gas causing it to expand and causing the gas excess volume to be spilled through the vent. When the sun sets, the lifting gas cools and contracts, as a result the volume of the balloon is reduced, which in turn reduces the lift. To keep the balloon afloat through the night, ballast must be dropped to arrest the descent. The balloon then climbs until the balloon hull is again fully extended. At each diurnal cycle, more lifting gas and ballast are lost. This depletion of lifting gas and ballast limits the duration of zero-pressure balloon flights to at most a few days when the ballast is exhausted. The one exception is flights at the poles during the summer, where there is sunlight for 24 hours a day, these flights can last for many more days.
Super-pressure balloons use hulls made of relatively high strength material that can contain the lifting gas at a pressure higher than the surrounding atmosphere. These balloons are filled with enough gas to expand the hull completely at the lowest temperature that would be encountered during the flight. Unlike the zero-pressure balloons, no vent and ballast are required for extended flights. During the day when temperature of the gas increases, it produces an increase in the internal pressure of the balloon. The hull must be capable of handling the large stresses produced by this pressure increase. These super-pressure balloons are relatively expensive to manufacture due to the high strength hull material needed and tight fabrication tolerances required. These balloons can produce extended flight duration, but the failure rate has been relatively high limiting their widespread use.
In addition, there have been reports of certain dual-balloon type systems. These may include, e.g., a tandem type balloon, where the balloons are situated above and below a given payload. In addition, there have been reports of double-envelope type balloons, where a super-pressure balloon is contained within a zero-pressure type balloon, where the volume of the zero-pressure balloon is greater than the contained super-pressure balloon. In this configuration, the relatively small super-pressure balloon is used for controlling altitude while the relatively large zero-pressure balloon is used for lifting the payload.
Reference is also made to U.S. Pat. No. 7,469,857 entitled “System and Method For Altitude Control.” According to the abstract this disclosure provides a differential expansion system and method for balloon buoyancy control which includes a zero-pressure envelope and a super-pressure envelope.
Accordingly, there remains a need for balloon systems that might offer a combination of the benefits of a zero-pressure balloon (e.g. the use of relatively low strength ductile film as a balloon material) but which would otherwise provide the longevity of flight duration super-pressure balloon (e.g., relatively long flight duration at targeted altitudes in fluctuating temperature conditions).